At the present time, the phosphoric acid anodising (PAA) process [1] is widely used in America
for the pretreatment of aluminium alloys used in adhesively bonded structures. With the
replacement of the Forest Products Laboratory (FPL) etch in the anodising line with a
hexavalent chromium free alternative [2], this has solved many of the environmental problems
associated with the ever increasing regulations enforced by both national and local authorities.
However, this route has never been favoured in the European aircraft industry due to the
superior bond durability offered by chromic acid anodising (CAA) relative to PAA in corrosive
environments. Furthermore, CAA has been shown to have twice the anodising throwing power
and generally higher peel strengths to that of PAA [3].